Polyacetal resin has various excellent properties and molded articles thereof are utilized in wide-ranging fields, but it has such nature that it is likely to decompose under a heated oxidizing atmosphere, or acidic or alkaline conditions because of the chemical structural features thereof. Accordingly, as the subject for polyacetal resin, it is mentioned to give it high thermal stability to suppress the generation of formaldehyde during the molding process or from the molded articles thereof. When the thermal stability is low, the polymer decomposes through heat treatment in such a process as extrusion or molding, thus resulting in the generation of adhering matter to the mold (mold deposit), or the lowering of the moldability or mechanical properties. In addition, formaldehyde generated by the decomposition is chemically active and forms formic acid by oxidation to cause adverse effects on the thermal resistance of the polyacetal resin, or when polyacetal resin generating a large quantity of formaldehyde is used for components and the like of electric/electronic devices, formaldehyde generated or formic acid being the oxidized product thereof may be factors for causing the corrosion of metallic contact parts, or discoloration or contact error due to the adherence of an organic compound. Although a very small quantity of formaldehyde is generated from polyacetal resin molded articles under usual use conditions, the generated formaldehyde itself constitutes one of causes for contaminating work environment in component-assembling processes or use environment of final products.
Consequently, in order to stabilize polyacetal resin, an antioxidant or other stabilizers are compounded. Regarding the antioxidant to be added to polyacetal resin, there are known phenolic compounds having steric hindrance (hindered phenol), amine compounds having steric hindrance (hindered amine) and the like. As the other stabilizers, melamine, polyamide, alkali metal hydroxide, alkali earth metal hydroxide and the like are used. In addition, usually, the antioxidant is used in combination with another stabilizer. However, only by compounding such general-purpose stabilizer to polyacetal resin having ordinary formaldehyde quality, it is difficult to drastically reduce formaldehyde generated, particularly, formaldehyde generated from molded articles. Further, there are disclosed polyacetal resin compositions having been compounded with various compounds in order to solve the above problem and reduce the generation quantity of formaldehyde. For example, there are disclosed a polyacetal resin composition containing polyacetal resin and a glyoxydiureide compound [JP-A 10-182928 (claim 1)], a polyacetal resin composition containing polyacetal resin and a cyclic nitrogen-containing compound (glycocyamidine such as creatinine or derivatives thereof) [JP-A 11-335518 (claim 1)], a polyacetal resin composition containing polyacetal resin, at least one processing stabilizer selected from polyalkylene glycol, fatty acid ester, fatty acid amide and fatty acid metal salt, and at least one inhibitor selected from urea or derivatives thereof and amidine derivatives [JP-A 12-26704 (claim 1)], a polyacetal resin composition constituted of polyacetal resin, a hindered phenol-based compound, a spiro compound having a triazine ring, and at least one selected from a processing stabilizer and a heat resistance stabilizer [JP-A 2003-113289 (claim 1)], and a polyacetal resin composition obtained by compounding polyaectal resin with a guanamine derivative such as benzoguanamine as a stabilizer [JP-A 62-190248], and the like.
Further, JP-A 2005-112995 discloses a polyacetal resin composition constituted of a polyacetal copolymer with a specified terminal group and a formaldehyde inhibitor and, as the formaldehyde inhibitor, guanamine compounds, urea-based compounds, hydrazide carboxylate-based compounds and the like.
On the other hand, in order to improve the weathering resistance of polyacetal resin, JP-A 61-36339 discloses a composition obtained by adding a benzotriazole-based material or the like and a hindered amine-based material to polyacetal resin to coexist. Further, JP-A 6-256623 discloses a composition for which weathering (light) resistance and thermal stability have been improved by compounding an antioxidant, tetrakis[methylene-3-(3′,5′-di-t-butyl-4′-hydroxyphenyl)-propionate]methane, a light stabilizer, bis-[N-methyl-2,2,6,6-tetramethyl-4-piperidinyl sebacate, and a UV absorber, 2-[2′-hydroxy-3′,5′-bis-(α,α-dimethylbenzyl)phenyl]-benzotriazole.
According to the techniques disclosed in JP-A 10-182928 (claim 1), JP-A 11-335518 (claim 1), JP-A 12-26704 (claim 1), JP-A 2003-113289 (claim 1), JP-A 62-190248 and JP-A 2005-112995, it is possible to significantly reduce the generation of formaldehyde from polyacetal resin, and according to the techniques disclosed in JP-A 61-36339 and JP-A 6-256623, it is possible to impart excellent weathering resistance to polyacetal resin.
Only a simple combination of these techniques can not bring, however, the additive property of effects into existence. In particular, when a hindered phenol and a hindered amine are used in combination, an opposing effect lowers thermal stability and light stability, thereby making it very difficult to obtain a resin substance having an excellent weathering resistance, significantly suppressing the generation of formaldehyde, and having no such problem as defective appearance due to the exudation of compounded components.